In the past, the semiconductor industry used various different device structures and methods to form semiconductor devices such as, for example, diodes, Schottky diodes, Field Effect Transistors (FETs), High Electron Mobility Transistors (HEMTs), etc. Devices such as diodes, Schottky diodes, and FETs were typically manufactured from a silicon substrate. Drawbacks with semiconductor devices manufactured from a silicon substrate include low breakdown voltages, excessive reverse leakage current, high drain to source resistance (Rds(on), unsuitably poor switching characteristics, low power densities, and high costs of manufacture. To overcome these drawbacks, semiconductor manufacturers have turned to manufacturing semiconductor devices from compound semiconductor substrates such as, for example, III-N semiconductor substrates, III-V semiconductor substrates, II-VI semiconductor substrates, etc. Although these substrates have improved device performance, they are fragile and add to manufacturing costs. Thus, the semiconductor industry has begun using compound semiconductor substrates that are a combination of silicon and III-N materials to address the issues of cost, manufacturability, and fragility. A III-N compound semiconductor material formed on a silicon substrate or other semiconductor substrate has been described in U.S. Patent Application Publication Number 2011/0133251 A1 by Zhi He and published on Jun. 9, 2011, and in U.S. Patent Application Publication Number 2013/0069208 A1 by Michael A. Briere and published on Mar. 21, 2013.
Semiconductor manufacturers have used a combination of silicon semiconductor materials and III-N semiconductor materials to manufacture devices, such as a normally-on III-N depletion mode HEMT cascoded with a silicon device. Using this combination of materials helps achieve a normally-off state using a III-N depletion mode device that is normally-on. In cascoded devices configured as switches, the silicon device often operates in avalanche mode due to high leakage currents of the III-N device operating under a high drain bias. In the past, silicon trench MOSFETs have used a shielded gate architecture in which a shield electrode is disposed in a trench underlying a gate electrode. Typically, the shield electrode is connected to the source electrode of the MOSFET and extends into the N-type drift region of the MOSFET. The shield electrode is separated from the drift region by a shield dielectric. Under reverse bias, the shield electrodes assist in depleting the N-type drift region between two adjacent trenches. This type of structure enables a higher doping in the N-type drift region and hence a lower Rds(on). The spacing between two adjacent trenches and the doping of the N-type drift region are optimized so that the drift region is fully depleted at the breakdown voltage of the device. Further, to promote high reliability and high avalanche energy capability, the impurity material concentration of the drift region is optimized such that the impact ionization occurs near the bottom of the trench. One consequence of this is that under avalanche conditions, there may be charge injection into the shield dielectric near the bottom of the trench which impacts the electric field in the drift region and causes the breakdown voltage of the MOSFET to decrease, resulting in device failure in poor reliability. In addition, in the avalanche operating mode, the gate of the III-N device is under a large stress in which the absolute gate to source voltage exceeds the devices pinch-off voltage. Hard stress conditions such as operating the silicon device in the avalanche mode degrades device reliability, lowers the breakdown voltage, and increases leakage currents. Although cascoded semiconductor devices have been described in U.S. Patent Application Publication Number 2013/0088280 A1 by Rakesh K. Lal et al. and published on Apr. 11, 2013, techniques for protecting the silicon field effect transistor and the gate of the III-N device have not been addressed.
Accordingly, it would be advantageous to have a cascoded semiconductor device structure and a method for manufacturing the cascoded semiconductor device that include clamping structures for protecting the silicon field effect transistor. It would be of further advantage for the structure and method to be cost efficient to implement.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference characters in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current flow through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-channel devices, or certain N-type or P-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with embodiments of the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action and the initial action. The use of the words approximately, about, or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to about ten percent (10%) (and up to twenty percent (20%) for semiconductor doping concentrations) are regarded as reasonable variances from the ideal goal of being exactly as described. For clarity of the drawings, doped regions of device structures are illustrated as having generally straight-line edges and precise angular corners; however, those skilled in the art understand that due to the diffusion and activation of dopants, the edges of doped regions are generally not straight lines and the corners are not precise angles.
Furthermore, the term “major surface” when used in conjunction with a semiconductor region or substrate means the surface of the semiconductor region or substrate that forms an interface with another material, such as a dielectric, an insulator, a conductor, or a polycrystalline semiconductor. The major surface can have a topography that changes in the x, y and z directions.